1. Field of the Invention
This invention relates to an electrophotographic process, and more particularly to an improved process for use in an electrophotographic reproducing apparatus, in which a photosensitive member is used, the photosensitivity of which in an image exposing step varies according to its prior history.
2. Prior Art
U.S. Pat. No. 3,494,789 discloses a photosensitive member, in which photoconductive particles, CdS.n CdCO.sub.3 (0&lt;n.ltoreq.4), consisting of cadmium sulfide and cadmium carbonate are dispersed in an electrically insulting resin binder for use in electrophotography. Such a photosensitive member will be referred to as a CdS.n CdCO.sub.3 resin photosensitive member, hereinafter.
The CdS.n CdCO.sub.3 resin photosensitive member as disclosed in U.S. Pat. No. 3,494,789 provides various advantages such as (i) a high photosensitivity equivalent or superior to that of vitreous selenium and the like, (ii) a high thermal stability, (iii) low pre-exposing effect, (iv) a long service life or endurance for repeated and continuous use, (v) a simple manufacturing process, and (vi) the feasibility to use a photosensitive member with either a positive or negative polarity.
However, tests by the inventors reveal that the CdS.n CdCO.sub.3 resin photosensitive member manufactured according to the process disclosed in the aforementioned patent results in a stained image. Various tests and experiments were made to clarify the reasons responsible for the stained image. As a result, it was found that a CdS.n CdCO.sub.3 resin photosensitive member affords a unique hysteresis effect. This unique hysteresis effect is such that: (i) when the charge and/or amount of exposure to be imparted to the photosensitive member prior to the charging and exposing steps are varied, then the photosensitivity of the member varies in the subsequent exposing step; (ii) the photosensitivity of the photosensitive member in an exposing step, where the member has been subjected to the charging and exposing steps after the pre-exposing of the photosensitive surface of the member which has a surface-potential of zero volts, is the same as that of the photosensitive member in an exposing step, where the member has been subjected to an exposing step after the pre-exposure of the photosensitive surface thereof, which affords a high surface potential at an exposure of no less than 10.sup.3 lx-sec. In other words, the photosensitivity of the member in an exposing step, where the member has been pre-exposed at an exposure value of no less than 10.sup.3 lx-sec, is not affected by the prior history of the pre-exposing step. Additionally, this hysteresis effect lasts for several seconds to several minutes, irrespective of positive or negative charging.
As has been described earlier, when a CdS.n CdCO.sub.3 resin photosensitive member is used in an ordinary reproducing process, then only stained images are obtained. This will be described in more detail hereinafter.
With a drum having a diameter on the order of 60 mm used as a photosensitive member, the circumference of the drum is approximately 190 mm, so that upon reproduction of a general copy size, for instance, A4 size (210.times.297 mm), the same portion of the photosensitive member is repeatedly used for a single copy sheet. For instance, in case A4 size sheet is used lenghwise, then about 60% of the surface of a photosensitive member is repeatedly used. When a CdS.n CdCO.sub.3 resin photosensitive member is used as a photosensitive member, then stained-image phenomena result, such as fogging in the forward half of the first copy sheet, which corresponds to the first cycle of rotation of the photosensitive member when reproduced by shifting a reproducing apparatus from its long rest condition to its operating condition. This phenomenon is referred to hereinafter as a foggy effect. There also results a phenomenon, in which the preceding copy image doubly appears on the rear half of the copy sheet, i.e., on the surface of the copy sheet which corresponds to a doubly-used surface of the photosensitive member. This phenomenon is referred to hereinafter as a memory effect.